Ice accumulation on surfaces consists of two treatable areas. The first is ice removal (deicing) which occurs by application of various deicing fluids or thermal or mechanical methods. The present invention accomplishes a second different function of ice prevention (anti-icing) the merits of which can be found on the ground as well as with in-flight aerospace use as well as with other surfaces of structures able to benefit by protection from freezing precipitation and other foul weather icing events. The purpose of this invention is to lower the freezing point of the metal surfaces on aircraft, rocket boosters and engine casings, as well as ground structures susceptible to ice accumulation from single and repeated icing events, such as power transmission towers, satellite tracking stations, metal equipment framework, cables, etc. The anti-icing function is accomplished by placing an aqueous freeze-point depressant solution in the absorbent fraction of a film suitably bonded to the surface of a structure so as to allow it to be maintained under adverse conditions of freezing rain, ice, snow, wind, air speed, temperature and time.
Fluids used for deicing and anti-icing typically are comprised of a blend of water and ethylene glycol or propylene glycol, in a ratio that ranges from 50:50, water to glycol, to about 20:80. They serve to lower the freezing point of water in the same manner that similar solutions provide anti-freeze protection in automobiles. For the purpose of this application the terms deicing fluid, anti-icing fluid and freeze-point reduction or depressant fluid will be used interchangeably. These fluids are sometimes diluted with water in the end use to match the weather conditions. Deicing fluids melt the frost, snow, or ice which has accumulated on, for example, aircraft surfaces, while the aircraft is on the ground, as well as provide protection against further accumulation and/or refreezing when no further precipitation occurs. Commonly sprayed on under pressure, the fluid melts the ice and snow while the force of the spray clears the surfaces. In the case of an aircraft, the formation of ice would change the aerodynamic flow characteristics of, for example, the wing of the aircraft and prevent its normal functioning. When these fluids are thickened, they provide an extended period of protection against frost, snow, and ice, i.e., an extended holdover time while the aircraft is on the ground, by remaining on the aircraft until take-off, and come off the surfaces when the aircraft becomes airborne. Such fluids are represented by U.S. Pat. No. 4,744,913, Salvador, et al., U.S. Pat. No. 5,118,435, Nieh, U.S. Pat. No. 5,461,100. Jenkins, U.S. Pat. No. 5,653,054, Savignano, et al. and U.S. Pat. No. 5,708,068, Carder, et al.
Deicing or anti-icing fluids have been classified as two types: unthickened and thickened. Unthickened deicing fluids are generally classified as Type I fluids, and are comprised of a blend of water and ethylene glycol or propylene glycol, in a ratio of about 20:80, water to glycol. They melt the frost, snow, or ice which has accumulated on the aircraft surfaces while the aircraft is on the ground. However, they do not provide adequate protection from further ice and snow formation. They mainly provide protection against refreezing when no further precipitation conditions occur.
Thickened deicing or anti-icing fluids, which have a ratio of about 50:50, water to glycol, are classified as either Type II or Type IV deicing fluids. They prevent ice and snow from forming on aircraft surfaces that remain on the ground for longer periods of time before take-off. Because they are thicker and more viscous, they remain on the aircraft until take-off rather than tending to flow off relatively soon after application as with Type I fluids. The thickened fluid is applied on to the aircraft surfaces after snow and ice have been removed, and snow and ice will then form on the coating, not on the aircraft surfaces. The thickened deicing coating is then removed from the aircraft by the shearing action during take-off when the aircraft reaches about 70 m/sec or 157 miles per hour. In neither case is long term protection against ice formation provided, even during flight, because the Type I through Type IV fluids eventually flow off of or are removed from the aircraft surfaces during take-off by airflow over the surfaces.
Deicing and anti-icing protection during flight have typically been provided by various means such as directed flow of engine exhaust or heat, electrical heaters built into or bonded to aircraft surfaces, or by providing a surface on an aircraft wing which can be selectively inflated and deflated to break up any ice formation. All of these add weight and complexity to aircraft systems and, in some cases, require that aerodynamic surfaces be reengineered to accommodate changes in airflow which cause changes in lift characteristics. Such mechanical, electrical and pneumatic ice removal systems are represented by U.S. Pat. No. 5,813,631, Butler, et al., U.S. Pat. No. 6,194,685, Rutherford and U.S. Pat. No. 6,352,601, Ray.
What is needed is a means to provide anti-icing protection to surfaces, particularly aircraft surfaces, which is lightweight and simple to apply and does not require additional complex mechanical and electrical means. A preferred form would be a coating which can be applied to surfaces as needed and which is highly absorbent of standard deicing and anti-icing solutions and has the ability to retain such fluids within the absorbent material under high load or wind conditions, i.e., while in flight, to provide extended time anti-icing for such surfaces.